The radar apparatus is configured to radiate an electric wave from a measurement point toward a space, and receive a reflected wave reflected by the target to measure a range and a direction from the measurement point to the target. In particular, in recent years, there has been advanced the development of the radar apparatuses which can detect not only automobiles but also pedestrians as the targets by measurement with high resolution using electric waves short in wavelength such as microwaves or millimeter waves.
The radar apparatus receives received signals in which the reflected waves from a short-range target and a long-range target are mixed together. In particular, in the case where a range sidelobe occurs due to autocorrelation characteristics of the signal of the reflected wave from the short-range target, the range sidelobe is mixed with the signal of the reflected wave from the long-range target when the radar apparatus receives the signals. Accordingly, a detection precision of the long-range target in the radar apparatus may be deteriorated.
Also, when the automobile and the pedestrian are present at the same distance from the measurement point, the radar apparatus may receive the received signal in which the reflected waves from the automobile and the pedestrian different in radar cross section (RCS) are mixed together. It is said that the radar cross section of the pedestrian (person) is lower than the radar cross section of the automobile. For that reason, even if the automobile and the pedestrian are present at the same distance from the measurement point, there is a need to appropriately receive the reflected waves from not only the automobile but also pedestrian.
Accordingly, there is a need to transmit a pulse wave or a pulse modulated wave having characteristics (hereinafter referred to as “low range sidelobe characteristics”) in which the range sidelobe level becomes low during transmission to the above-mentioned radar apparatus where the high-resolution measurement is required. Further, the radar apparatus is required to provide a wide receiver dynamic range for the received signal.
For the above-mentioned low range sidelobe characteristics, a pulse-compression radar has been known up to now, which transmits a complementary code as the pulse wave or the pulse modulated wave having the low range sidelobe characteristic. The pulse compression represents a technique in which a signal wide in the pulse width in which a pulse signal is subjected to pulse modulation or phase modulation is transmitted, and in signal processing after receiving the signal, the received signal is demodulated and converted into a signal narrow in the pulse width. With the pulse compression, a sensing range of the target can be increased, and a distance estimate precision in the sensing distance can be improved.
Also, the complementary code includes a plurality of, for example, two complementary code sequences (an, bn). In an autocorrelation operation result of one complementary code sequence and an autocorrelation operation result of the other complementary code sequence, the complementary code has a property that the range sidelobe becomes zero when delay times (shift times) τ are matched with each other to add the respective autocorrelation operation results together. A parameter n is n=1, 2, . . . , L, and a parameter L represents a code sequence length.
The property of the above complementary code will be described with reference to FIG. 9. FIG. 9 is an illustrative view illustrating the property of a conventional complementary code. FIG. 9(a) is an illustrative view illustrating the autocorrelation operation result of one complementary code sequence an. FIG. 9(b) is an illustrative view illustrating the autocorrelation operation result of the other complementary code sequence bn. FIG. 9(c) is an illustrative view illustrating an added value of the autocorrelation operation results of the two complementary code sequences (an, bn).
The autocorrelation operation result of one complementary code sequence an of the two complementary code sequences (an, bn) is derived from Expression (1). The autocorrelation operation result of the other complementary code sequence bn is derived from Expression (2). A parameter R represents the autocorrelation operation result. When n>L or n<1 is satisfied, both of the two complementary code sequences (an, bn) are set to zero. An asterisk * represents a complex conjugate operator.
                    [                  Ex          .                                          ⁢          1                ]                                                                                  R            aa                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                          ⁢                                    a              n                        ⁢                          a                              n                +                t                            *                                                          (        1        )                                [                  Ex          .                                          ⁢          2                ]                                                                                  R            bb                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                          ⁢                                    b              n                        ⁢                          b                              n                +                t                            *                                                          (        2        )            
As illustrated in FIG. 9(a), a autocorrelation operation result Raa(τ) of one complementary code sequence an derived from Expression (1) is peaked when the delay time τ is zero, and has the range sidelobe when the delay time (shift time) τ is not zero. Likewise, as illustrated in FIG. 9(b), a autocorrelation operation result Rbb(τ) of the other complementary code sequence bn derived from Expression (2) is peaked when the delay time τ is zero, and has the range sidelobe when the delay time τ is not zero.
As illustrated in FIG. 9(c), an added value of those autocorrelation operation results (Raa(τ), Rbb(τ)) is peaked when the delay time τ is zero, and becomes zero without any range sidelobe when the delay time τ is not zero. This is represented by Expression (3). The axis of abscissa in FIGS. 9(a) to 9(c) represents the delay time τ in the autocorrelation operation, and the axis of ordinate represents the autocorrelation operation result.[Ex. 3]Raa(τ)+Rbb(τ)≠0, when τ=0Raa(τ)+Rbb(τ)=0, when τ≠0  (3)
Also, a pulse compression transmitting/receiving device and a pulse compression transmission/reception method disclosed in Patent Literature 1 have been known as the above-described conventional pulse compression radar using pulse compression.
In Patent Literature 1, the transmitting device transmits a pulse having a width T which has been subjected to pulse phase modulation in one of the complementary sequences, and a pulse having the width T which has been subjected to pulse phase modulation in the other complementary sequence, at a time interval w which is equal to or larger than the transmission pulse width T. Further, the transmitting device transmits the double transmission pulses every PRI (pulse repetitive interval) after having receiving the reflected pulses of those double transmission pulses. A correlator of the receiving device obtains a correlation between a signal of the first time interval T of the received signal of a time interval (2T+w), and a reference signal that has been modulated by one complementary sequence used for modulation of a first transmission pulse.
Further, the correlator of the receiving device obtains a correlation between the signal of the last time interval T, and a reference signal that has been modulated by the other complementary sequence. A determiner of the received signal determines a correlation value according to the two correlation results obtained by the correlator. As a result, the range sidelobe where the received signal is not subjected to phase modulation attributable to the Doppler frequency is set to zero, and the deterioration of the range sidelobe level where the received signal is subjected to the phase modulation attributable to the Doppler frequency can be reduced.
Also, there has been known a property that the attenuation of the signal of the reflected wave long in arrival time from the long-range target is larger than that of the reflected wave short in the arrival time from the short-range target from the measurement point. In order that the radar apparatus has a large receiver dynamic range, there has been known that the signal of the reflected wave is amplified by an AGC (auto gain control) unit with respect to the reflected wave from the target in association with the above-mentioned property. This will be described with reference to FIG. 10.
FIG. 10 is an illustrative view illustrating the operation of amplifying the signal of the reflected wave in the conventional radar apparatus. FIG. 10(a) is an illustrative view illustrating a transmission zone of the transmission signal and a measurement zone of the received signal. FIG. 10(b) is an illustrative view illustrating a change in a gain that is amplified by the AGC unit within the measurement zone of the received signal.
FIG. 10(a) illustrates an example in which the measurement zone of the reflected wave of the pulse signal is provided in a zone corresponding to a non-transmission zone when the pulse signal is intermittently transmitted in a transmission period including the transmission zone and the non-transmission zone. In this case, as illustrated in FIG. 10(b), the gain of the AGC unit is increased more as an elapsed time from the non-transmission zone is longer. As a result, the large receiver dynamic range can be realized in the radar apparatus.